In today's business and scientific world color has become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of digital color print engines are continuously looking for ways to improve the accuracy and total image quality of their products.
Color images are commonly represented as one or more separations, each separation comprising a set of color density signals for a single primary or secondary color. Color density signals are commonly represented as digital gray or contone pixels, varying in magnitude from a minimum to a maximum, with a number of gradients between corresponding to the bit density of the system. Thus, a common 8 bit system provides 256 shades of each primary color. A color can therefore be considered the combination of magnitudes of each pixel, which when viewed together, present the combination color Usually, printer signals include three subtractive primary color signals (Cyan, Magenta and Yellow) and a Black signal which together can be considered the printer colorant signals. Each color signal forms a separation and when combined together with the other separations, forms the color image.
Printers commonly provide a limited number of output possibilities, and are commonly binary, i.e., they produce either a dot (ink drop) or no dot at a given pixel location. Thus, given a color separation with 256 shades of each additive primary color, a set of binary printer signals must be produced representing the contone effect. This process is referred to as halftoning. In such arrangements, over a given area and the separation having a number of contone pixels therein, each pixel value of an array of contone pixels within the area is compared to one of a set of preselected thresholds (the thresholds may be stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell) as taught for example in U.S. Pat. No. 4,149,194 to Holladay. The effect of such an arrangement is that, for an area where the image is a contone, some of the thresholds in the matrix will be exceeded, i.e., the image value at that specific location is larger than the value stored in the dither matrix for that same location, while others are not. In the binary case, the pixels or cell elements for which the thresholds are exceeded might be printed as Black or some color, while the remaining elements are allowed to remain white or uncolored, dependent on the actual physical quantity described by the data. Since the human visual system tends to average out rapidly varying spatial patterns and perceives only a spatial average of the micro-variation in spot-color produced by a printer, the halftone process described above can be used to produce a close approximation to the desired color in the contone input.
The dither matrix of threshold values is often referred to as a “screen”, and the process of generating the binary image from the contone image using the screen is called “screening”. Conventional digital halftones start as a number of isolated dots which grow bigger as more colorant is requested on the paper. These screens are referred to as clustered-dot screens. The fundamental rate at which the dots in a clustered dot screen are repeated is commonly referred to as the screen's spatial frequency [Note R. Ulichney, “Digital Halftoning”, MIT Press, Cambridge, Mass., 1987]. The higher the screen spatial frequency, the finer and smoother appearing the image and also the greater is the capacity for the dots to represent fine detail in the image. Dithering creates problems in color document reproduction where the repeating pattern of a screen through the image, when superposed over similar repeating patterns in multiple separations, can cause moiré or other artifacts, particularly in a printing system with less than ideal registration between separations.
Stochastic, or non-periodic screening is an alternative to conventional clustered dot screens. Instead of producing dots that grow with increased colorant on paper, the stochastic screening method produces a well-dispersed pattern of isolated dots at spaced pixel locations. Thus there is no fundamental periodicity in the dots, instead the design of the screen attempts to produce patterns with pleasant noise characteristics. The pleasant noise characteristics are achieved by designing the screen so as to distribute the noise energy in the region of high spatial frequency, where the human visual system has a significantly reduced sensitivity. In this respect, U.S. Pat. No. 5,673,121 to Wang, discloses a stochastic halftone screening method for designing an idealized stochastic screen and is herein incorporated by reference as it discloses a particular stochastic screen useful in implementation of the subject invention, as will be more fully explained below. One of the advantages of stochastic, or non-periodic screening over periodic screening, is the suppression of moiré.
Color stochastic screening has typically been implemented in the prior art by using the same screen for all separations (dot-on-dot) or by using independent screens (possibly obtained by shifting/rotating/flipping a single screen). These methods do not produce halftones with maximal ink dispersion and optimized spatial frequency response because there is no control of interseparation overlaps. The less successful the screening processes, the less smooth, i.e. more grainy, the resultant image appears. It is of course an overall objective of the subject invention to produce a resultant image in which the graininess is minimized, smoothness is enhanced and the intended colors are accurately reproduced. Related methods have been proposed for error diffusion and screening in U.S. Pat. No. 6,072,591 and in U.S. Pat. No. 5,631,748 to Harrington, and in the publication “Color Stochastic Screening with Smoothness Enhancement”, by J. Shu, C. H. Li, H. Ancin, and A, Bhattarcharjya, IS&T's NIP13: 1997 International Conference on Digital Printing Technologies.
Another problem relates to the smooth reproduction of highlight regions which is a challenge for most halftone printers. A highlight region is typically a color which is fairly close to white, e.g., a pale sand or pale gray color. Bi-level halftoning for highlight regions is particularly susceptible to graininess problems. In order to reduce visibility of halftone textures in highlight regions, a number of printing technologies utilize multiple printing levels per colorant. This is achieved either by using light and dark inks or by using a small drop-size and allowing multiple drops per pixel (multi-level halftoning). The lighter inks/smaller drops reduce the amount of luminance variation produced by the printed dots and thus produce highlight regions that have less visible halftone texture than those produced with darker colorants or larger drops. For photo-quality inkjet printing, this is increasingly becoming the predominant mode of printing as compared to bi-level halftoning and has been a strong driver of the evolution of inkjet printing technology.
While multi-level halftoning offers an inherent benefit over bi-level halftoning in reducing luminance variation in the highlights, the halftone texture visibility is still strongly influenced by the interactions between the different printed separations. Halftoning the different separations using the same halftone screen in a “dot-on-dot” configuration gives rise to the largest possible variation in luminance and therefore the most visible halftone textures. The more typical configuration of independent halftone screens for the different separations performs better but still exercises only limited control over inter-separation overlap.
Accordingly, there is a need for a halftoning algorithm for multi-level halftoning that utilizes a single halftone screen but achieves improved control of the interaction between halftone separations by utilizing a successive-filling algorithm for multi-level halftoning. The algorithm should give rise to smoother halftone textures and better ink dispersion than what is achieved with don-on-dot or independent screens